The present invention pertains generally to lasers and more particularly to stimulated Raman scattering utilizing rotational transitions in a diatomic molecular gas.
The present invention comprises an improvement over U.S. Pat. No. 4,222,011 issued Sept. 9, 1980, to Norman A. Kurnit entitled "Stokes Injected Raman Capillary Waveguide Amplifier" and application Ser. No. 229,023 filed Jan. 27, 1981 by Norman A. Kurnit entitled "A Ring cavity For A Raman Capillary Waveguide Amplifier." The disclosures of the above referenced patent and application are hereby incorporated by reference.
As disclosed in the above referenced patent, various methods have been disclosed for shifting frequencies of conventional lasers in the ir spectrum. These methods have included four wave mixing as disclosed in U.S. Pat. No. 4,095,121 by Richard F. Begley et al., entitled "Resonantly Enhanced Four Wave Mixing," and "Raman Scattering," as disclosed in U.S. Pat. No. 4,061,921 by C. D. Cantrell et al. entitled "Infrared Laser Systems" and reissue application Ser. No. 967,171 filed Mar. 16, 1979 by C. D. Cantrell et al., entitled "Infrared Laser System," now U.S. Pat. Re. No. 30,898. In each of these systems and other previous systems of ir frequency shifting to a broad range of frequencies, simplicity and overall efficiency are important factors for economic utilization of the device.
Since this stimulated Raman effect can be produced in a single step with high conversion efficiency, Raman shifting of CO.sub.2 laser radiation provides high overall efficiencies due to the high efficiencies and well developed technology of CO.sub.2 lasers. However, Raman gain in gaseous media such a H.sub.2, D.sub.2, HD, HT, DT, or T.sub.2, require powers which are near the breakdown threshold of these diatomic molecular gases for a single pass focus geometry such as suggested by Robert L. Byer in an article entitled "A 16 Micron Source for Laser Isotope Enrichment" published in IEEE Journal of Quantum Electronics, QE-12, pp 732-733, November 1976.
The above-referenced U.S. Pat. No. 4,222,011 by Norman A. Kurnit discloses a capillary waveguide amplifier and regenerative amplifier which utilize a Stokes injection source to reduce the required field strength of the CO.sub.2 laser radiation and eliminate the necessity for spontaneous generation of Stokes radiation within the capillary waveguide amplifier. However, in a single pass waveguide amplifier configuration, high output energies are not always obtainable due to the limited output power of the Stokes injection source. The use of a regenerative amplifier, such as disclosed in FIG. 2 of the above-referenced patent, fails to overcome the problems of the single pass geometry since the power of the Stokes injection source is limited by the dichroic mirrors utilized in the regenerative system and since the only significant gain produced in the capillary waveguide amplifier is achieved when the Stokes signal is copropagating with the CO.sub.2 laser pump radiation in the forward direction. Although it is possible to reflect back the CO.sub.2 radiation so that both the Stokes and CO.sub.2 laser pump signal copropagate in both the forward and reverse directions in the regenerative capillary waveguide amplifier, the field intensity may be increased beyond the breakdown threshold of the Raman scattering medium gas. When this occurs, no gain can be achieved.
Additionally, since the Stokes source in many applications has a pulse width much smaller than the CO.sub.2 laser pulse width, only a small fraction of the CO.sub.2 laser energy can be extracted in a system such as disclosed in the above-referenced U.S. Pat. No. 4,222,011. Consequently, it would be desirable to extract energy over a broader range of the CO.sub.2 laser pulse width.
The above-referenced U.S. patent application Ser. No. 229,023, filed by Norman A. Kurnit entitled "A Ring Cavity For A Raman Capillary Waveguide Amplifier," of which the present invention comprises an improvement, discloses a ring cavity configuration for extracting energy from a latter portion of the CO.sub.2 pulse so as to provide better temporal overlap. Additionally, the Stokes signal copropagates with the CO.sub.2 laser pulse around the ring cavity to thereby provide complete spatial overlap. The combination of these factors allows a much greater portion of the CO.sub.2 laser pulse energy to be extracted.
However, it is also desirable to convert the very early and very late part of the CO.sub.2 pulse to the Stokes frequency, particularly if the CO.sub.2 laser is run in a long pulse mode in order to increase the CO.sub.2 laser energy extraction efficiency. In this case, it may be desirable to compress the energy in the long CO.sub.2 pulse into a shorter Stokes pulse by means of backward stimulated Raman scattering. It is well known that, despite the generally lower gain for counterpropogating pump and Stokes, a strong Stokes pulse is capable of virtually complete depletion of the pump which passes through it in the Raman-active medium. Thus, long pump pulses can be converted to short duration Stokes pulses of much higher peak power by passing them in opposite directions through a Raman medium whose length is one half the sum of the spatial extents of the two pulses.
Although the ring cavity configuration extracts a large portion of the energy from the CO.sub.2 pulse and requires a much smaller Stokes source in the amplifier configuration, the ring cavity configuration is not necessarily capable of pulse compression and virtually complete extraction of energy from the CO.sub.2 pulse.